(I use Firefox. While there may be another way to zoom in & out, the control & + or - works there. I thought that might be common.)

I use Google Chrome and the ctrl- and ctrl+ both work fine for zooming out and in respectively.
Edit: I just reread your suggestion: "try hitting control & - at the same time" cunninglinguist probably used the & as well?
Next edit: Now I don't think so. I first used ctrl + and ctrl - using the numeric keypad. When I used the + at the top of the keybrd I had to use ctrl shift + to zoom in and ctrl shift - to zoom out.

Spike: sounds like you use keybrd shortcuts like I do. I used them extensively when I used to design spreadsheets 20 years ago. So much faster and no need to constantly take your hand off the keybrd to operate the mouse. To this day it drives me crazy to see people paging up or down 1 at a time or scrolling with a mouse wheel to get to the top of a window. Ctrl home gets you there in 1 keystroke combo. Ctrl A is great too for selecting everything instead of clicking and then dragging a mouse for seconds at a time.

lalatan, I use some keyboard shortcuts, but not all. (I'm lazy, but sometimes too lazy to learn the shortcuts.) Anyway, you're right about the cntl+ & cntl-. I always use the numeric keypad for that, not the normal + & - keys. Having to press three keys defeats the purpose of a shortcut as far as I'm concerned.